In an aircraft with mechanical flight controls, the pilot manoeuvres the machine by means of controls in the cockpit, such as the yoke or rudder pedals, connected mechanically, typically by cables, to a set of control surfaces adapted to modify the trajectory of the machine. The pilot generally controls yaw by means of control surfaces on the axial vertical tail fin controlled by the rudder pedals. Lateral acceleration, generally denoted Ny, generated for example by a change of engine operating conditions on a twin-jet or turbo aircraft or by evolving flight conditions (wind, turbulence, lowering of gear, change of speed, etc.) leads to laterally asymmetrical flight. This induced asymmetry is uncomfortable for passengers and critical for the pilot, in particular in the event of failure of an engine.
With no automatic yaw axis control, the pilot seeks throughout the flight to limit side-slip and must constantly adapt the trim on the roll and yaw axes. A trim control enables reduction or even elimination of the force to be exerted on one of the main attitude controls of the aircraft (pitch via the yoke, roll via the yoke or yaw via the rudder pedals). Most often there is a small control surface at the edge of the main control surface that is actuated in the opposite direction, thereby generating a moment to balance the moment from the main control surface and thus cancel out these forces.
Accordingly, a prior art aircraft with mechanical flight controls includes in the cockpit mechanical controls for yaw axis control, generally two rudder pedals, operating on a system of cables connected to the yaw control surface. It also includes a trim device which can be activated in the cockpit and which acts via a trim actuator on the compensator of the main control surface, also known as the trim tab. In flight, the pilot must be attentive and correct lateral forces by acting on the rudder pedals and the trim control. This imposes a heavy workload, notably on jet or turbo aircraft and when changing engine operating conditions. Moreover, it is difficult to ensure perfect symmetry of the aircraft, resulting in passenger discomfort and excess fuel consumption. In the event of failure of an engine, the pilot must moreover react very quickly to control completely movement about the yaw axis. The risk of asymmetrical stalling is high during these critical flight phases.